Generally, an exhaust gas which is discharged from a diesel engine contains a contaminant. More specifically, the contaminant may be nitrogen oxides (NOx) and a particulate matter (PM).
The above mentioned contaminant contaminates the atmosphere such that the contaminant is required to be purified before being discharged to the atmosphere. According to the related art, a technique which purifies the contaminant using an exhaust gas post-processing apparatus is known.
The exhaust gas post-processing apparatus of the related art is provided with a diesel oxidation catalyst (DOC) and a selective catalytic reduction (SCR) in an exhaust gas line.
The diesel oxidation catalyst (DOC) removes carbon monoxide (CO) and hydrocarbon (HC) contained in the exhaust gas and oxidizes nitrogen oxides (NO) in the exhaust gas to be nitrogen dioxide (NO2).
The selective catalytic reduction (SCR) sprays a reducing agent (urea aqueous solution) to the exhaust gas to mix the exhaust gas and the reducing agent. Thereafter, a thermal decomposition process is performed under a high temperature environment to reduce nitrogen oxides into nitrogen and water vapor.
During this process, the reducing agent (urea aqueous solution) is excessively sprayed in a low temperature operating area where an activity of the SCR catalyst is low so that the reducing agent is deposited in an exhaust gas pipe and a mixer to be solidified. However, the reducing agent and the exhaust gas are not uniformly mixed due to irregular flowing of the exhaust gas caused by an excessively generated deposit. Therefore, targeted efficiency of converting the nitrogen oxides is lowered and a back pressure due to the deposit is increased, which results in lowering the performance of the engine.
Therefore, in order to maintain the performance of the selective catalytic reduction (SCR), a regeneration operation which removes a reducing agent which is deposited in the pipe to be solidified needs to be performed.
According to the regeneration operation, a temperature of the exhaust gas is increased up to hundreds degrees Celsius so that the solidified reducing agent may be thermally analyzed. As a method for increasing the temperature of the exhaust gas, it is known to increase an engine RPM or perform an overloading job. Further, a fuel is sprayed to an upstream of the exhaust gas post-processing apparatus to forcibly increase the temperature of the exhaust gas.
The above-described regeneration operation may be performed according to a set schedule or forced regeneration may be manually performed in accordance with an intention of an operator.
Further, when a specific environment is developed, the regeneration operation is automatically performed. For example, when an increased back pressure of the exhaust gas is detected, as described above, the temperature of the exhaust gas is increased.
However, even though the regeneration operation is performed according to a set schedule or the forced regeneration is performed in accordance with the intention of the operator, when the environment where the regeneration is necessary is developed, the regeneration needs to be automatically performed.
A pressure sensor is provided on the exhaust gas line, more specifically, at the upstream of the diesel oxidation catalyst (DOC) and it is checked whether a back pressure is formed using the pressure. However, when the pressure sensor is not normally operated, the automatic regeneration is not performed.
A background art of the exhaust gas post-processing apparatus is disclosed in the following Patent Document.